This invention relates to coal beneficiation and more particularly relates to a coal beneficiation method useful for cleaning coal wherein the coal particle size distribution is in the range of from about 37 microns to about 600 microns. It has been known in the prior art that various methods are used to clean coal having particle sizes from about 6300 microns to approximately 600 microns, that is to produce low ash and low sulfur fuels from coal of this size. In addition, some of these techniques have been applied to coal down to 150 microns in size with limited success.
However, except for froth flotation, no satisfactory commercial process exists for satisfactorily cleaning finer coal, that is in the range of from about 37 to about 150 microns.
Coal beneficiation has been useful to make coal a high quality, more flexible, and desirable fuel for new uses such as coal-water mixtures for utility or industrial boilers, dry particulate or slurry fuels for diesel or gas turbine applications, and for conventional applications such as electric utilities and export.
Producing such low-ash and low-sulfur fuels for these applications requires complete or near complete liberation of mineral matter from the coal. By mineral matter, it is intended to include ash as well as sulfur-bearing pyrite. Most often, this degree of liberation is realized only at particle sizes of less than 150 microns. However, few techniques exist for treating such ultrafine sizes of coal, and on a commercial basis, froth flotation has historically been the only technique used for cleaning fine coal down to a 37 micron fineness. Despite its widespread use, froth flotation is inefficient, especially at the finer particle sizes and is particularly poor or inefficient in rejecting pyrite from the cleaned coal product. This inherent inability to remove pyrite by the froth flotation process has become a severe detriment as concern for high sulfur emissions into the environment has increased.
Magnetite (Fe.sub.3 O.sub.4) is widely used in dense-medium gravimetric processes. It is mixed with water to form a suspension with a relative density between 1.2 and 2.0--a dense medium. In general, commercial grades or size distributions of magnetite are used for beneficiation. For instance, grade E magnetite is about 95% less than 45 microns with about 25% less than 5 microns. Finer magnetite has not been used to clean coal successfully on a commercial scale. One South African article reported tests with finer magnetite, but they were unable to recover the magnetite at an acceptable level or to clean coal down to 37 microns.